Persistent pain is a significant clinical and public health problem. Once established, persistent pain is often refractory to existing treatments and associated with adverse health outcomes. While substantial advances have been made in our knowledge of nociceptive pathways, our current understanding does not explain variations in the susceptibility of individuals to the development of persistent pain. A key epigenetic mechanism is the regulation of chromatin structure through histone posttranslational modifications (PTM)s. Histone PTMs enable a cell to alter its gene expression by modifying protein interactions with specific DNA sequences. Gene expression varies from silent to active depending on the amount and types of histone PTMs present in regulatory regions and throughout the length of the gene. Inflammatory and neuropathic pain models are associated with altered levels of histone PTMs in the peripheral nerve, dorsal root ganglion (DRG), spinal cord, and in regions of the brain and that injury-induced histone PTMs changed gene expression. The central hypothesis of this proposal is that the histone PTM state of gene promoters and enhancers in DRG cells is associated with altered gene expression and maintenance of persistent pain. The specific aims of this proposal are: 1) to determine epigenetic changes in pain-related genes by comparing injury- and inflammation-induced histone PTMs in the rat DRG, and 2) to determine the associations between histone PTMs and mRNA expression in candidate pain-related genes and identify the DRG cells responsible for these changes. The proposed studies will use chromatin immunoprecipitation followed by massively parallel sequencing (i.e., ChIP- seq) to perform a genome-wide comparison of histone PTM patterns in the L4-L6 ipsilateral DRG from rats in a widely used animal model of neuropathic pain induced by chronic constriction injury of the sciatic nerve and in a model of persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant. Gene enhancers and promoters that show similarly altered PTM levels in both models will be designated as pain- related genes. Quantitative PCR will be used to measure mRNA expression in the DRG and the cellular localization of the mRNA will be determined using in situ hybridization. Identification of epigenetic mechanisms involved in the maintenance of persistent pain promises to further elucidate biologic mechanisms, predict pain outcomes following trauma or disease, and facilitate the development of novel therapies.